GB 5009.123-2023National food safety standard - Determination of chromium in foods (English PDF)

3.1 Reagents

3.1.1 Nitric acid (HNO3).

3.1.2 Perchloric acid (HClO4).

3.1.3 Ammonium dihydrogen phosphate (NH4H2PO4).

3.2 Preparation of Reagents

3.3 Reference Material

Potassium dichromate (K2Cr2O7, CAS No.. 7778-50-9). purity > 99.99%, or a standard

substance certified by the state and awarded a reference material certificate.

3.4 Preparation of Standard Solutions

3.4.1 Chromium standard stock solution (1,000 mg/L). accurately weigh-take 0.2829 g of

potassium dichromate (110 C, bake for 2 h), dissolve it in water, then, transfer into a 100 mL

volumetric flask. Use nitric acid solution (5 + 95) to dilute it to the scale and evenly mix it. The

mass concentration of chromium in this solution is 1,000 mg/L.

4.1 Atomic absorption spectrometer. equipped with graphite furnace atomizer and chromium

hollow cathode lamp.

4.2 Electronic balance. with a division value of 0.1 mg and 1 mg.

4.3 Adjustable electric heating plate or adjustable electric stove.

4.6 Constant-temperature drying oven.

4.7 Muffle furnace.

5.1 Specimen Preparation

5.1.1 Solid-state samples

5.1.1.1 Dry samples

For samples with low-water content, such as. beans, grains, fungi, tea, dried fruits and baked

goods, etc., take the edible part, when necessary, use a high-speed pulverizer to evenly crush it.

For powdery samples that are uniform in shape, such as. solid dairy products, protein powder

and flour, etc., evenly shake them.

5.1.2 Liquid samples

For samples, such as. soft drinks and condiments, etc., evenly shake them.

5.1.3 Semi-solid samples

Evenly mix them.

5.2 Test Conditions of the Method

5.2.1 Specimen pre-treatment

5.2.1.1 Wet digestion method

For solid specimens, weigh-take 0.2 g ~ 3 g (accurate to 0.001 g). For liquid specimens,

accurately transfer-take or weigh-take 0.500 mL (g) ~ 5.00 mL (g) (accurate to 0.001 g) into a

digestion tube with scale.

5.2.1.2 Microwave digestion method

For solid specimens, weigh-take 0.2 g ~ 0.5 g (accurate to 0.001 g; for samples containing

relatively high-water content, the sampling size can be appropriately increased to 1 g). For

liquid specimens, accurately transfer-take or weigh-take 0.500 mL (g) ~ 3.00 mL (g) (accurate

to 0.001 g) into a microwave digestion tank. For samples containing ethanol or carbon dioxide,

firstly heat them at a low temperature to remove ethanol or carbon dioxide, add 5 mL ~ 10 mL

of nitric acid, in accordance with the operation procedures of microwave digestion, digest the

specimen.

5.2.1.4 Dry digestion method

For solid specimens, weigh-take 0.5 g ~ 5 g (accurate to 0.001 g); for liquid specimens,

accurately transfer-take or weigh-take 2.00 mL (g) ~ 10.0 mL (g) (accurate to 0.001 g) into a

crucible, and heat it over low heat; carbonize it, until it becomes smokeless. Then, transfer it

into a muffle furnace, at 550 C, ash it for 3 h ~ 4 h.

5.3 Reference Conditions of Instruments

See Table A.2 for instrument operating conditions.

5.4 Drawing of Standard Curve

In accordance with the mass concentration from low to high, respectively take 10 L of the

standard series of solutions and 5 L of the ammonium dihydrogen phosphate solution (select

the optimal sample injection volume in accordance with the instrument used), meanwhile, inject

it into the graphite tube. After atomization, determine the absorbance value, and by taking the

mass concentration as the x-coordinate and the absorbance value as the y-coordinate, draw a

standard curve.